The present invention relates to timing clocks and more particularly to an improved technique for providing long term stability to delay line clocks.
Generally, delay line clocks are utilized in systems wherein it is desired to produce an output timing pulse which is coherently related to the time of initiation of the delay line clock. In particular, in distance measuring systems, such as radar systems, it is necessary to measure time intervals beween a pulse transmission and target echo in order to determine particular target characteristics. The rate at which the pulses are generated is known as the pulse repetition frequency (PRF) and it is to this rate that the receiving system must be synchronized in order that such characteristics as target range may be accurately measured. While delay line clocks are capable of providing requisite timing waveforms capable of being synchronized to the pulse repetition frequency, the same are susceptible to instabilities when operated for long periods of time. If the radar transmitter were constrained to transmit at an exact instant of time, an incoherent crystal clock could provide the necessary timing in relation to the pulse repetition frequency. However, where it is desired to operate the transmitter asynchronously, there are presently no techniques which enable the accuracy of the crystal oscillator to be utilized as a timing waveform. While various other techniques have been proposed to provide the necessary correlation, the same utilize complex equipment which has been burdensome in cost and system performance. There is therefore a continuing need for clock synchronizing circuitry which would enable the use of the less complex and less costly delay line clock while still providing the necessary long term stability for more versatile operation in current radar systems.
Accordingly, the present invention has been developed to overcome the specific shortcomings of the above-known and similar techniques and to provide a circuit for allowing frequency corrections to a delay line clock on a sampled basis.